1. Field of the Invention
The present invention relates to compositions for controlling the gelation rates of water soluble polymer solutions used as fracturing fluids in well fracturing operations and, more particularly, but not by way of limitation, to suspensions of boron sources, an organophilic clay, and a non-aqueous solvent. The suspension provides a stable, non-aqueous, pumpable, source of borate ions suitable for use as a gelling agent for water soluble polymer solutions.
2. Description of the Related Art
To perform a hydraulic fracturing operation, a fracturing fluid comprising a proppant-laden water soluble polymer solution of guar, hydroxypropyl guar (HPG), carboxymethyl guar (CMG), or carboxymethyl hydroxypropyl guar (CMHPG) is injected under high pressure into a formation through a well bore. Once the natural confining pressures of the formation rock are exceeded, the fracturing fluid initiates a fracture in the formation rock that generally continues to grow during pumping. Hydraulic fracturing of the formation typically requires the fracturing fluid to reach its maximum viscosity as it enters the fracture. Increased viscosity of the fracturing fluid, which improves its capacity to fracture the formation rock, usually occurs through the gelling of the water soluble polymer solution utilized as the hydraulic fracturing fluid. Gelation of water soluble polymer solutions is typically achieved by the addition of aluminum, boron, titanium or zirconium ions, or mixtures thereof to the polymer solution.
However, if the fracturing fluid gels within the well bore, it encounters a high shear due to the limited cross-sectional area within the well bore. High shear experienced in the well bore may cause extensive and irreparable degradation to the cross-linked fracturing fluid. Furthermore, high viscosities in the fracturing fluid produce excessive back or friction pressures within the well bore and formation, thereby limiting the pumping rate and possibly the success of the hydraulic fracturing operation. Various borate ion cross-linking systems have been developed which delay the gelation of the fracturing fluid during its pumping through the well bore.
One such borate ion cross-linking system is disclosed in U.S. Pat. No. 4,619,776 issued on Oct. 28, 1986, to Mondshine. Mondshine discloses the use of alkaline earth metal borates. The boron minerals disclosed are sparingly soluble in the water soluble polymer solutions. After the introduction of the Mondshine borate formulation into a water soluble polymer solution, the borate source(s) slowly dissolves to release the borate ions, which gradually cross-link the polymer solution. That is, the slow solubility of the sparingly soluble borate minerals creates a cross-linking system that delays the transformation of polymer solutions into gelled, highly viscous fracturing fluids.
Although the borate ion cross-linking system disclosed by Mondshine delays the gelation of water-soluble polymer solutions, it suffers from several disadvantages. The cross-linking delay is determined by the solubility of the sparingly available borate. This limited solubility of the borate source impedes controlling the cross-linking delay due to the scarcity of borate ions. This can only be provided by modifying the base formulation of the product. For example, a system requiring a delay of X minutes may need a formulation A, while a system requiring a delay of Y minutes may need a different formulation B. Thus, the Mondshine system fails to provide variable delay control from a single product formulation. Furthermore, the Mondshine system is restricted to formations having bottom hole temperatures below about 230.degree. F. (110.degree. C.), indeed Mondshine suggests the incorporation of organometallic crosslinking agents to enhance performance of the fluid system above 275.degree. F. (135.degree. C.). Additionally, the Mondshine system experiences the potentially negative effect of other components in the borate source, e.g. calcium or magnesium ions, interacting with the polymer and, thus, interfering with the crosslink reaction.
An alternative borate ion cross-linking system is disclosed in U.S. Pat. Nos. 5,082,579; 5,145,590; and 5160,643; which issued to Dawson. These patents disclose a borate ion based aqueous complexor solution that delays the gelation of water soluble polymer solutions. The aqueous complexor solution consists of a cross linking additive that provides borate ions and a delay additive in solution that serves to chemically bond with the borate ions to reduce the availability of boron to the hydrated polymer in solution.
Although the Dawson borate ion cross-linking system provides improvements to some of the deficiencies characteristic of the Mondshine system, such as lack of control over the gelation rate, the Dawson system still suffers from a number of disadvantages. The amount of available borate ions in solution may be sufficient to cross link all of the polymer in solution due to the presence of the delay additive. The dilution of borate ions caused by the presence of the delay additive creates a demand for considerably higher quantities of borate ions than required in a stoichiometrically balanced system. The requirement for additional boron in excess of that necessary for a stoichiometrically balanced system increases the cost of the Dawson borate ion cross linking system. Another disadvantage is that manufacturing the Dawson cross-linker is lengthy and burdensome. A further disadvantage of the Dawson system is caused by the undesirable necessity of having to use large quantities of carbonate buffers, such as potassium carbonate, to obtain effective delay and temperature stability of the fracturing fluid, which is length of time the crosslinked fluid maintains viscosity above a given minimum at a given temperature and shear rate on a Model 50 viscometer. Furthermore, due to the presence of the delay additive in solution with the borate ions, the cross-linking action of the complexor solution is thermally delayed, i.e., the onset of crosslinking or gelation does not occur until a certain temperature is reached. The thermal delay of the Dawson system thus restricts use of that system to formulations having bottom hole temperatures higher than about 125.degree. F. Finally, and perhaps most importantly, during storage, the borate ions in the Dawson complexor solution may precipitate rendering the complexor solution deficient in boron.
Another borate ion cross-linking system is disclosed in U.S. Pat. No. 5,488,083 issued on Jan. 30, 1996, to Kinsey, et al. The Kinsey system utilizes an anhydrous boron compound suspended in a mineral spirits resin solution. The resulting suspension is added to a water soluble polymer solution to achieve a cross-linked fracturing fluid. Although the Kinsey cross-linking system provides improvements to some of the deficiencies of the prior art, such as controllable delay and high concentration of boron content in the cross-linking agent, this system still suffers from several disadvantages. The cross-linking agent is extremely water sensitive, in that the cross-linker forms a cake or sludge in the presence of water, thereby making the product difficult to handle in the field due to the potential for contamination from pumps or lines that have had water in them. The Kinsey system also has operability limitations. During transport to field locations, settling may occur and re-suspension requires more than simple stirring, and the use of air wands or circulating pumps increases the chance of water contamination. Additionally, the Kinsey system is difficult and costly to dilute, thereby impeding metering at low treatment rates that demand low cross-linker concentrations.
A further borate ion cross-linking system is disclosed in U.S. Pat. No. 5,372,732 issued on Dec. 13, 1994, to Harris et al. The Harris system discloses a cross-linking agent that contains a borate source compounded with a water soluble polysaccharide. The Harris system provides a borate cross-linking agent that is convenient to use at a well site, however, the Harris system is expensive to manufacture and provides poor control of the cross-linking reaction.
Still another borate ion cross-linking system is disclosed in U.S. Pat. No. 3,974,077, which issued on Aug. 10, 1976 to Free. The Free cross-linking system includes a borate source and a water-soluble polymer solution. A basic compound, such as sodium hydroxide or magnesium oxide, raises the pH of the fluid to begin cross-linking. The Free system, however, suffers from a lack of control of the cross-linking delay. Gelation is often too rapid where sodium hydroxide is used to raise the pH often too slow where magnesium oxide is used to raise the pH. Moreover, the Free system is inflexible in using variable concentrations of fluid alkalinity. Fluid alkalinity is a key factor for the performance of the borate cross-linked fluid. Lack of fluid alkalinity flexibility impedes adjusting the process to improve cross-linking performance. Furthermore, a strong alkali is needed to increase the performance of the cross-linked fluid and Free's use of sodium hydroxide does not provide for controllable gelation. Magnesium oxide is a weak alkali, which diminishes the performance of the borate cross-linked fluid. Without modifications to address the presence of magnesium where magnesium oxide is used to facilitate the gelation of the borate cross-linked fluid, the formation of magnesium hydroxide at certain elevated temperatures limits the temperature stability of the fluid.
Accordingly, a borate ion cross-linking system that provides a controllable cross linking delay; provides a high content and supply of borate; is easy to manufacture, transport, and use at the field site; and is economical will improve over other currently available borate cross-linking system.